In general, a voltage regulator is a circuit that is designed to maintain a constant output voltage level as operating conditions change over time. Electronic circuits are designed to operate with a constant DC supply voltage. A voltage regulator circuit provides a constant DC output voltage and contains circuitry that continuously holds the output voltage at the desired value regardless of changes in load current or input voltage (assuming that the load current and input voltage are within the specified operating range for the regulator). Maintaining accurate voltage regulation is particularly challenging when the load current variations are sudden and extreme, e.g., minimum load to maximum load demand in less than a couple hundred picoseconds. Such sudden and extreme variations in load current can occur in applications in which the circuitry being powered by the regulator is primarily CMOS logic. Since the majority of the current drawn by CMOS logic is dynamic (current that is used to charge and discharge parasitic capacitances) and not static (such as DC leakage currents), the load current presented to the regulator can change from a minimum to a maximum very quickly when the CMOS logic switches from an idle state to a state with high activity factor (maximum workload).
Linear voltage regulators are the most commonly used types of voltage regulators in integrated circuits (ICs) and have a number of advantages. Linear voltage regulators are fully integrable, requiring no off-chip components such as inductors. Unlike switching types, linear regulators generate no inherent ripple of their own, so they can produce a very “clean” DC output voltage, achieving low noise levels with minimal overhead (cost). Typically, a linear regulator operates by modulating the voltage drop across a series pass element, which can be modeled as a voltage-controlled resistance. The control circuitry monitors (senses) the output voltage. If the output voltage is lower than desired, a voltage is applied to the series pass element which decreases its resistance; since less voltage is dropped across the series pass element, the output voltage rises. Similarly, if the output voltage is higher than desired, the resistance of the series pass element is increased, so more voltage is dropped across the series pass element, and the output voltage falls. Since the output voltage correction is achieved with a feedback loop, some type of compensation is required to assure loop stability.
Most linear regulators have built-in compensation and are completely stable without external components. The need to maintain adequate loop stability (phase margin) limits the achievable bandwidth of linear regulators. Hence, any linear regulator requires a finite amount of time to correct the output voltage after a change in load current demand. This “time lag” defines the characteristic called transient response, which may not be fast enough for applications with sudden and extreme load current variations, such as with CMOS logic applications as noted above.
Another drawback of the limited loop bandwidth of linear regulators is that it is hard to achieve good power supply rejection ratios (PSRR) at high frequencies (e.g., 100 MHZ-1 GHz). Finally, the power efficiencies of linear regulators with even moderately fast transient responses tend to be low since significant static current is consumed in the wideband amplifier stages used to drive the series pass element.
A different approach to realizing a regulator capable of fast response to sudden changes in load current is to use a high-speed comparator as the primary error detector controlling the conduction of the series pass element. In particular, one type of voltage regulator which has very fast transient response characteristics is referred to as a “bang-bang” type voltage regulator, in which a high speed comparator is utilized to switch a series passgate element from fully on to fully off (and vice versa). The fast response time makes bang-bang type voltage regulators more suitable than their linear counterparts to handle highly varying load current demands with minimal effect on regulated voltage and with the capability of providing nearly instantaneous response to any variation in load current demand. The fast response time also improves the high-frequency power-supply rejection ratio (PSRR).
However, the use of bang-bang regulators poses design challenges with regard to the ability to achieve suitable DC accuracy on the regulated voltage (due to offsets of the high-speed comparator) and to limit the intrinsically generated ripple on the regulated output that results from the sudden switching of the passgate current (bang-bang operation). Another problem arises when a distributed regulator system is formed by connecting the outputs of multiple bang-bang regulators to a common supply grid, as even small mismatches in comparator offsets may result in highly unequal sharing of the load current.